1. Field of the Invention
This invention relates to check valves which may be used in fluid lines, for example feedwater supply lines for steam generators, to prevent significant reverse flow of feedwater from the steam generator in the event of abnormal reduction of pressure in the feedwater line.
2. Description of the Prior Art
In many industrial processes it is important that reverse flow in the fluid lines be prevented in the event of loss or reduction of pressure. One such application is in the feedwater lines in steam generating systems. In order to prevent damage to the steam generator and to the pumps which supply the feedwater to the steam generator, it is important that any reverse flow from the steam generator back towards the pump be prevented in the event there is a drop in pressure in the feedwater line which might be caused, for example, by a failure of one or more of the supply pumps or by a rupture in the feedwater supply line. Unless means are provided to quickly respond to any tendency for reverse flow, serious damage can be caused to either the steam generator or the feedwater pumps. Check valves such as that disclosed in U.S. Pat. No. 4,046,164 are conventionally used to provide this protection. As described in that patent, such valves are comprised of a dashpot controlled valve element which is held open by the fluid pressure when the flow is in the normal direction and at normal pressure. However, any significant drop in pressure in the inlet to the valve will be sensed by the valve and will cause it to automatically close to prevent any reverse flow. While it is desirable that such valves should respond quickly to any such drop in pressure, it is also important that the rate of closure be controlled to prevent the valve from impacting too heavily on the valve seat thereby damaging either the valve or the seat or both after one or more closure operations. Also, uncontrolled closure of the valve at too high a speed may also cause damaging pressure surges due to "water hammer". A closure time in the order of one second will provide this requisite protection against reverse flow which will prevent excessively large impacts on the valve seat and pressure surges. The dashpot is provided to control the rate of closure to prevent such impacting, all as described in U.S. Pat. No. 4,046,164.
In the valve therein described, the escape of fluid from the dashpot chamber is primarily through an annular space between the orifice plate and the piston rod of the dashpot or through special passages through the orifice plate. In either case, the fluid bleed passage is fixed in area and is sized to provide the requisite closure time, i.e., in the order of one second, under the most extreme conditions, namely, a line rupture which could cause a complete loss of pressure in the feedwater line upstream of the valve. It will be appreciated that under these conditions the pressure differential applied to the orifice plate of the dashpot is greatest.
One of the problems encountered in such a valve with a fixed orifice area is that when less extreme conditions are encountered, i.e., when there is a less drastic drop in the pressure in the feedwater line, such a valve may take an excessively long time to close. Another problem encountered with such valves, particularly those valves which utilize a controlled clearance between the piston rod and the orifice plate, is that such clearance may be adequately controlled in the larger sizes of valves, such as those from 12 inches or above but in the smaller sizes below that range such a clearance is difficult to control with any degree of precision in order to provide the requisite degree of uniformity of performance from valve to valve.